The application generally relates to a method of operation and apparatus for noise attenuation of positive displacement compressors. The application relates more specifically to a method of operation and apparatus for noise attenuation of screw compressors that decreases the peak to peak amplitude and increases the frequency of the composite pressure pulse of the screw compressors by varying the speed of one or more of the screw compressors. The invention accomplishes noise attenuation without the use of an error sensor in the discharge line or through the use of an acoustic sensor thereby reducing the complexity of the noise reduction system. In addition, by reducing the peak-to-peak amplitude and increasing the frequency of the composite pressure pulse the muffler system is reduced in both size and cost.
Heating and cooling systems are typically used to maintain temperature control in a structure. A primary component in such a system is a positive displacement compressor which receives a cool, low-pressure gas and by virtue of a compression device, exhausts a hot, high-pressure gas. One type of positive displacement compressor is a screw compressor, which generally includes two cylindrical rotors mounted on separate shafts inside a hollow, double-barreled casing. The sidewalls of the compressor casing typically form two parallel, overlapping cylinders which house the rotors side-by-side. Screw compressor rotors typically have helically extending lobes and grooves on their outer surfaces forming a large thread on the circumference of the rotor. During operation, the threads of the rotors mesh together, with the lobes on one rotor meshing with the corresponding grooves on the other rotor to form a series of gaps between the rotors. These gaps form a continuous compression chamber that communicates with the compressor inlet opening, or “port,” at one end of the casing and a compressor discharge opening or port at the opposite end of the chamber. The gas enters a continuous compression chamber at the inlet port and is continuously reduced in volume as the rotors turn thereby compressing the gas as the gas travels to the discharge port. Once the compressed gas reaches the discharge port it is provided to the rest of the system.
These rotors rotate at high rates of speed, and multiple sets of rotors (compressors) may be configured to work together to further increase the amount of gas that can be circulated in the system, thereby increasing the operating capacity of a system. While the rotors provide a continuous pumping action, each set of rotors (compressor) produces pressure pulses as the pressurized gas is discharged at the discharge port. These discharge pressure pulsations act as significant sources of audible sound within the system.
To eliminate or minimize the undesirable sound, noise attenuation devices or systems can be used. One example of a noise attenuation system is a dissipative or absorptive muffler system typically located at the discharge of the compressor. The use of muffler systems to attenuate sound can be expensive, depending upon the frequencies and peak-to-peak amplitudes that must be attenuated by the muffler system. Typically, the lower the frequency of the sound to be attenuated, and the greater the peak-to-peak amplitude the greater the cost and size of the muffler system.
What is needed is a system and/or method that satisfies one or more of these needs or provides other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments that fall within the scope of the claims, regardless of whether they accomplish one or more of the aforementioned needs.